Allow roles to be fetched with an array

For the case where the array may be stored in a variable, for example:

    set :my_roles, [:app, :web]

    on roles fetch(:my_roles) do
      #
    end

This will allow https://github.com/capistrano/rails/pull/30 to be
refactored
Merge pull request #803 from capistrano/feature/roles-array

Allow roles to be fetched with an array